When a material that is easily bent, such as, for example, a workpiece with a small diameter, is ground with a grinding wheel using superabrasive grains such as cubic boron nitride (CBN) grains or diamond grains, the distances among the abrasive grains are made large because the grinding force must be reduced. Specifically, as shown in FIG. 1, a filler composed of alumina (Al.sub.2 O.sub.3) is mixed with a bonding material (vitrified bonding material), and the superabrasive grains are retained with such a bonding material (vitrified bonding material) with suitable distances to uniformly disperse the superabrasive grains.
In the above-described prior art where alumina (Al.sub.2 O.sub.3) grains 20 as a filler are mixed with a bonding material 30 composed of the vitrified bonding material as shown in FIG. 1, since the alumina grains 20 composed of relatively small grains are uniformly present around the superabrasive grains 10, the distances among the superabrasive grains 10 at the grinding surface is determined by the grain size of the alumina (Al.sub.2 O.sub.3) grains 20 but cannot be widened as expected. Because the affinity between alumina and the vitrified bonding material is not high, the filler composed of the alumina grains 20 is easily dropped from the vitrified bonding material 30 in grinding work as shown in FIG. 2. As a result, the bonding force of the vitrified bonding material 30 is reduced by the dropping of the filler 20, and the bonding force to retain the superabrasive grains 10 is also reduced. Therefore, a problem arises where the superabrasive grains 10 are dropped in an early stage.